Document the secondary expansion method. Also, some other documentation

Document the secondary expansion method.  Also, some other documentation
cleanups.

If we find a make error (invalid makefile syntax or something like that)
write back any tokens we have before we exit.

If we have waiting jobs (using -j + -l) set an alarm before we sleep on
the read() system call, so we can wake up to check the load and start
waiting jobs, if there are long-running jobs we would otherwise be
waiting for.  Suggested by Grant Taylor.